rerere.c: diagnose a corrupt MERGE_RR when hitting EOF between TAB and '\0'

rerere.c: diagnose a corrupt MERGE_RR when hitting EOF between TAB and '\0'

If we reach EOF after the SHA1-then-TAB, yet before the NUL that
terminates each file name, we would fill the file name buffer with \255
bytes resulting from the repeatedly-failing fgetc (returns EOF/-1) and
ultimately complain about "filename too long", because no NUL was
encountered.

Signed-off-by: Jim Meyering <jim@meyering.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>